1. Field of the Invention
The present invention relates to an image forming apparatus, e.g. a copier, a facsimile device, or a printer for forming images on a transfer material, and specifically relates to an image forming apparatus for forming superimposed images.
2. Description of Related Information
Image forming apparatus which have a plurality of scanning units for scanning light images as shown in FIG. 11 are well known.
FIG. 11 is a cross sectional view of an image forming apparatus or color printer which has four drums to make full color images. This four drum type color printer has image forming stations 101C, 101M, 101Y and 101BK which respectively form color images, i.e., cyan, magenta, yellow and black images. These stations respectively have photosensitive drums 102C, 102M, 102Y and 102BK, optical scanning units 103C, 103M, 103Y and 103BK, chargers, developing units and cleaners, shown generally about the drums 102C, 102M, 102Y, and 102BK.
The four drum type color printer makes a cyan image, a magenta image, a yellow image and a black image on respective photosensitive drums. These images are sequentially transferred to a transfer material S which is transported along a direction A by a transportation belt 112. Thus, a plurality of color images are superimposed on the same surface of the same transferring material S. Therefore, if the transferred image position of each image forming station is shifted from a standard or correct position, different color images that should be superimposed relative to each other will not be resulting in false color images, and color hue would be different from the natural color hue in the case of natural full color images. Further misregistration can become so great that color misregistration is caused and the quality of image seriously deteriorates. As kinds of misregistrations, a misregistration in the transporting direction, a misregistration in the scanning direction, an inclination of scanning lines and an error of magnifications are known.
The misregistration in the transporting direction, which is called "a misregistration of top margin" in this specification thereafter, is apparent from FIG. 12(a). Namely, the top of the images in the transporting direction A of the transferring material S are misregistered with each other.
The misregistration of scanning direction, which is called "a misregistration of left margin" in this specification hereafter, is apparent from FIG. 12(b). Namely, the starting position of scanning of each color image is different from others thereof.
As shown in FIG. 12(c), the inclination of scanning line is caused by a difference of the angle which is the transporting direction of the transferring material S relative to the scanning line of each image forming station. And, as shown in FIG. 12(d), an error of magnification is caused by a difference between the magnifications of the scanning unit which is inherent from distances between the photosensitive drums and respective scanning mirrors.
Often, these four types of misregistrations do not happen independently, but happen simultaneously.
"The misregistration of top margin" is caused by a difference of the start timing of formation of images at the image stations 101C, 101M, 101Y and 101BK. "The misregistration of left margin" is caused by a difference of the start timing of scanning a the image stations 101C, 101M 101Y and 101BK.
The inclination of the scanning line is caused by inclination .theta..sub.1 of the scanning components or by inclination angle .theta..sub.2 between axes of the photosensitive drums 102C, 102M, 102Y and 102BK, as shown in FIG. 13 and FIG. 14 respectively. The error of magnifications is caused by a difference .DELTA.L between the scanning location of the components 103C, 103M, 103Y and 103BK and the photosensitive drums 102C, 102M, 102Y and 102BK as shown in FIG. 15 and FIG. 16.
In order to remove these four kinds of misregistrations, a number of proposals have been made.
For example, "the misregistration of top margin" and "the misregistration of left margin" can be removed by adjusting the start timings of optical scanning operation electronically.
On the other hand, the inclination of the scanning line and the errors of magnifications can be removed by mechanically adjusting the components to remove positional errors arising during installation and errors in the angle of the optical scanning by the components 103C, 103M, 103Y and 103BK and the photosensitive drums 102C, 102M, 102Y and 102BK. Namely, such adjusting is achieved by adjusting positions and angles of the scanning components, the photosensitive drums and the mirrors in the optical paths between the scanning components and the photosensitive drums.
The misregistrations of top margin and left margin are generated according over time, but they can electronically be adjusted. However, it is very difficult to adjust the inclination of scanning lines and the errors of magnifications, which are caused by misplacement of the optical scanners, the photosensitive drums or the mirrors in the path between the optical scanners and the drums. One of reasons for this difficulty is inherent in the high precision of the system, e.g. a diameter of one picture element would be only 62 .mu.m in the printer which has a resolution of 400 DPI (dot per inch). There are many other causes for generation of misregistrations, e.g., running states of the transferring belt (winding or inclining), replacement of the photosensitive drums, or movement of the machine. Also the timing signals for vertical synchronization and horizontal synchronization are so unstable that the misregistrations of the top margin and the left margin easily result.
Thus, after extended operating time of the system all kinds of misregistration take place. Therefore, it takes much time and much labor to adjust the system to reduce the misregistration. Further, adjustment equipment for all the image forming stations is necessary, so that cost of this type printer can become great.
Relative positions of a main body, optical systems, photosensitive drums 102C, 102M, 102Y and 102Bk and so on, adjusted when initially installed, may vary if the apparatus is moved from one place to another. Therefore, after movement of the apparatus, very complicated and difficult readjustment is required to prevent misregistration. Moreover, in such highly precise image forming apparatus, which does not compare with conventional image forming apparatus, variation in temperature also effects misregistration because of the thermal expansion and contraction of various components.
Adjustments of four misregistrations above mentioned must be performed on all image holding members or to all image forming stations. Therefore, it takes a long time to adjust the misregistrations. In particular, when the apparatus is operated in a monocolor mode, a two color mode or a triple color mode, there is at least one image forming station which is not used for an image formation. In spite of the non-use of this image forming station, registration marks of the non used station should be transferred and read, and the non-used station should be adjusted as well as in the full color mode. But such procedure is very wasteful. Therefore, time for adjusting in a one color mode can take as long as in a full color mode. Accordingly, efficiency of printing operation decreases. And the useful life of parts in the non used station, including the photosensitive drum, are shortened.